In an image forming apparatus such as an electronic copy machine, a facsimile, a printer, etc., an electrostatic latent image is formed on a photosensitive drum by scanning the surface of the drum with a laser beam emitted from a semiconductor laser such as a laser diode, thereby developing the latent image using a developer (toner) and transferring it onto a sheet of paper.
The diameter of a laser beam emitted from the laser diode is determined in accordance with the wavelength of the beam. The shorter the wavelength of the laser beam, the smaller the diameter of the laser beam. A laser beam of a short wavelength and a small diameter is suitable for high-speed, high-density image forming. A red laser beam is an example of a laser beam of a short wavelength and a small diameter.
The laser diode for emitting a red laser beam, however, has a high radiation efficiency. Accordingly, when a driving current in the form of a pulse wave has fallen, the temperature of the diode soon falls, whereas when the driving current has risen, the temperature does not promptly increase. A period of several microseconds to several tens of microseconds is necessary until a sufficient light output is obtained after the driving current rises. This delay in the rise of output light adversely affects image forming.